When untreated springs of the above-described types are subjected to heat treatment, as shown in FIG. 8 by way of example, springs are charged from the upper portion of a furnace body of cylindrical form and are heat-treated while flowing downwardly along a conveyance passageway formed as a vertically extending helix on the inner peripheral surface of the furnace body. The heat-treated springs are discharged from the lower portion of the furnace body.
However, since the conveyance passageway is formed to have a large width so as to smoothen the downward flow of the springs, there are cases where the springs contact the wall surfaces of the conveyance passageway or become entangled with one another in the course of their downward flow due to the longitudinal and transverse attitude of the springs during their downward flow through the conveyance passageway. In consequence, the downward flow speed of the springs becomes irregular. When the downward flow speed is too low, overheating results; when too high, heat treatment is insufficient. In other words, the problem that results is that the heat treatment cannot be performed uniformly.
In order for the aforementioned heat treatment to be carried out for a rquired, sufficient amount of time, it is necessary that the conveyance passagway have a certain length. Therefore, the conveyance passageway of the required length is formed on the inner peripheral surface of the abovementioned furnace body. When this is done, the furnace body is increased in vertical length since the conveyance passageway is so formed as to have a number of loops vertically along the inner peripheral surface of the furnace body.
As a result of the foregoing, the heat used in the treatment comes to reside solely in the upper region of the furnace body. Consequently, the temperature distribution within the furnace is such that the upper region becomes high in temperature while the lower region drops in temperature, resulting in a prounouced temperature difference. Accordingly, the springs which flow down into the lower region are not sufficiently heat-treated. Thus, one problem is that a satisfactory heat treatment cannot be carried out. Another is that the furnace body is large in size.